An Electric motor and spring tension mechanism was used by Arrow Dynamics Launched Loop coasters, which were popular in the 1970s and 1980s. A working example is Revolution at Blackpool Pleasure Beach.
Electric Motor and Spring Tension Launch Roller Coasters
A powerful electric motor and tensioned springs are used to propel a launch car forward. The launch car pushes the roller coaster train outward from the loading platform where it drops down a hill and through a loop. After exiting the loop the train climbs a hill leading to the opposite platform. When the train reaches the opposite platform, another catch car works the same way, this time pushing the train backwards.
A ride consisting of two elevated platforms separated by a single loop is referred to as a shuttle roller coaster.
Example of an Electric Motor and Spring Tension Launch